Piezoelectric devices are used in a variety of systems that include parts vibrating at a controlled frequency. These devices employ certain physical properties of piezoelectric crystals, also known as materials displaying the Polarized Electrostrictive Effect. In particular, piezoelectric crystals change their shape when exposed to voltage. Thus, by applying a driving circuitry to a piezoelectric material, the material can be adapted to expand and contract at the frequency associated with the driving circuit. This method of translating electrical energy to mechanical energy is used in actuators and transducers in a variety of fields.
One such field of application is optical temperature instrumentation. Generally speaking, non-contact temperature instruments allow measuring the temperature of an object at a distance and are quick to respond. These operating features are particularly helpful when measuring the temperature of an object in a harsh or dangerous environment where physical contact is not an option. Such instruments generally operate by sensing the energy emitted from objects at a temperature above absolute zero in which the radiant infrared energy emitted by the object is proportional to the fourth power of its temperature. To develop a measurement, some devices use a shield, often called a chopper, to alternately expose and block the target object to a sensor or a detector, thereby creating a modulated signal.
This function of optical modulation performed by choppers is one of the key aspects in the construction of opto-electronic sensors and in opto-electronic instrumentation in general. In optics, modulation can be used to counteract the imperfections of the circuitry, the detectors, and the medium, and further to distinguish the desired signal from the background.
A chopper used to modulate signals in an optical measurement device may be implemented in a variety of ways. There are motor-driven, opto-electronic, acousto-optical, and piezoelectric choppers. Piezoelectric choppers have the advantage of being very small in size and fairly inexpensive compared to other types of optical modulators. Moreover, as compared to other choppers, piezoelectric choppers are easier to assemble and have higher reliability. Meanwhile, motors used in motor-driven choppers are bulky and not very reliable due to the number of moving parts. Opto-electronic and acousto-optic choppers, on the other hand, have a different disadvantage of being very complicated, expensive, and often provide a relatively limited range in the so-called depth of modulation.
However, while piezoelectric choppers have the advantage of being small and simple, they also have a significant drawback. Specifically, commercially available piezoelectric choppers do not meet all of the desired performance requirements. For example, if one needs an apparatus requiring a tightly controlled modulation frequency which can operate consistently in a wide range of temperatures (between −10 C to 70 C, for instance), piezoelectric choppers will not work reliably because piezoelectric materials have a very large temperature coefficient. In other words, the electric properties of piezoelectric materials are highly sensitive to the ambient temperature. As a result, when a piezoelectric chopper is exposed to some variations in the ambient temperature, it demonstrates significant shifts in its resonant frequency, thus creating a problem in the task of achieving a reasonable stability in the signal modulation.
One of ordinary skill in the art will recognize that the response of a system will result in the largest possible amplitude when the corresponding circuit is driven at a resonant frequency associated with the system. For this reason, the manufacturers of piezoelectric choppers typically drive the circuits at the resonant frequency in order to obtain the maximum amplitude for the same applied voltage. As discussed above, the resonant frequency of a circuit involving piezoelectric materials is a function of the ambient temperature. Thus, commercially available choppers driven at the resonant frequency are inherently unstable at the modulation frequency selected as a function of the chopper temperature.
There has been an attempt in the industry to address some of the deficiencies of a piezoelectric chopper. In particular, the manufacturers sometimes deal with the problem of instability by adding a feedback sensor mechanism that obtains a measure of the modulation frequency. However, this method falls short of solving the problem for at least two reasons. First, the circuits using a feedback sensor loop introduce additional response time issues associated with the sensor feedback loop. Second, the addition of a sensor (coupler/interrupter) adds complexity to the system. Moreover, this additional subsystem typically has its own temperature coefficient which may actually compound the effect and fail to yield an accurate chopper control. In general, all feedback sensors, such as inductive, capacitive, and optical types, are sensitive to temperature and humidity. As a result, all feedback methods inherently have significant limitations.
As another alternative, chopper manufacturers sometimes choose not to deal with the temperature coefficient of piezoelectric materials and, as a result, simply do not assure the stability of the system. This lack of assurance may only be acceptable when piezoelectric choppers operate exclusively at a very narrow temperature range. However, many if not most of the industrial applications actually require instrumentation capable of working reliably within a wide range of temperatures such as 0° C. to 70° C.